1. Field of the Invention
The present invention relates to a ball screw rack assist type electric power steering apparatus.
2. Related Background Art
Steering systems of automobiles widely use a so-called power steering apparatus for assisting a steering operation by use of an external power source. the power source for the power steering apparatus has hitherto involved the use of a hydraulic pump, and this hydraulic pump is driven by the engine in many cases. This type of power steering apparatus has, however, a large power loss (on the order of several through ten horsepower (HP) at a maximum load) of the engine because of driving the hydraulic pump at all times and is therefore hard to apply to mini-sized motor vehicles exhibiting small displacements. Even the automobiles having comparatively large displacements, when installed, come to have such an inevitable demerit that their running fuel economy becomes low enough not to be negligible.
Such being the case, an electric power steering (which will hereinafter be abbreviated to EPS) involving the use of an electric motor as a power source has attracted the attention over the recent years for obviating those problems. The EPS apparatus exhibits characteristics, wherein there is no direct drive loss of the engine because of using a battery loaded into the car as a power source for the electric motor, a decrease in the running fuel economy is restrained as the electric motor is started only when assisting the steering operation, and in addition the precise electronic control can be extremely easily performed.
On the other hand, for a steering gear mechanism for a passenger car, a rack-and-pinion steering gear is mainly employed at the present, since the rack-and-pinion gear has high rigidity performance and low in weight. Then, the EPS apparatuses for the rack-and-pinion steering gear include a column assist type in which the electric motor is arranged sideways of the column in order to drive a steering shaft and a pinion itself, and in addition a ball screw rack assist type in which a rack shaft is driven by an electrically-driven ball screw mechanism. In the ball screw rack assist type EPS apparatus (which will hereinafter be simply called the rack assist type EPS apparatus), the assist force does not act on a meshing surface between the pinion and the rack, and therefore a contact surface pressure between these two members, which is to be a factor for abrasion and deformation as well, is relatively small.
According to the rack assist type EPS apparatus, a ball screw shaft male thread groove formed in a rack shaft engages with a female thread groove formed in a ball nut through a multiplicity of circulation balls (steel balls), and the ball nut is rotated by the electric motor, thereby moving the rack shaft in the axial directions. A power transmission method for transmitting the power from the electric motor to the ball nut may be a timing belt system disclosed in Japanese Utility Model Publication No. 6-49489 etc, however, a general method is a gear system disclosed in Japanese Utility Model Publication Nos. 5-14939 (a first prior art) and 2-46455 (a second prior art) etc. In case of the conventional rack assist type EPS apparatus, if low of an assembly accuracy of the respective parts constituting the ball screw mechanism and the rack and pinion mechanism, a misalignment occurs between the ball nut and the rack and pinion pair. In this case, so-called distortions are formed between the ball nut and the rack shaft, with the result that a smooth movement of the rack shaft is hindered. This hindrance of the smooth movement might cause a loss of driving force, an abnormal abrasion and, besides, noises when operated. Further, if the working accuracy and the assembly accuracy of the housings that accommodate and hold the ball screw mechanism and the electric motor, are low, a backlash occurs between the gear pair connecting the ball nut and the electric motor together, and the noises are likewise emitted.
Such being the case, each of the rack assist type EPS apparatuses according to the first and second prior arts adopts an aligning mechanism for supporting the ball nut so as to be swayable about the housing through a spherical bearing in order to obviate a misalignment between the ball nut and the rack shaft. Large increases in the number of manufacturing processes and in the costs for the product are, however, inevitable for providing the spherical bearing between the housing and the ball nut. Moreover, the spherical bearing utilizes the slide on between the metal members, and hence, if a comparatively large load acts, a predetermined aligning operation is not obtained due to a rise in slide friction force, with the result that the misalignment is not obviated in the great majority of cases. Further, if the ball nut is supported on the spherical bearing, a central position of the ball nut remained unchanged, so that the backlash between the gears is not, as a matter of course, canceled.
Generally the ball nut configuring the ball screw mechanism is structured so that a rolling bearing (a double-row angular ball bearing etc) is fitted on and made integral with the ball nut body. The ball nut is fixed by a ring bolt or a C-shaped stopper ring brought into contact with an outer ring of the rolling bearing within the housing.
In the conventional rack assist type EPS apparatus, the C-shaped stopper ring comes off and falls out of an engagement groove formed in the housing, or the ring bolt slackens and falls out due to a careless operation in the assembly and vibrations when driving. In this case, since the ball nut moves in an axial direction by dint of an external force, a proper axial force can not be given to the rack shaft. This results in an extremely unagreeable feeling of the steering and in an impossibility of performing the precise steering.
According to the rack assist type EPS apparatus adopting the gear type power transmission method, an idle gear is interposed between the gear on the electric motor and the gear on the ball nut in order to avoid an increase in diameter of each of these two gears.
In the rack assist type EPS apparatus, the gear train for the power transmission, the ball screw mechanism and the bearing for supporting the ball nut, have hitherto been lubricated by respective lubricants (normally greases) for exclusive uses therefor. Hence, in this type of EPS apparatus, three or four pieces of shaft seals are attached to side portions etc of the gear portion of the ball nut, thereby preventing the lubricants from being mixed. If many shaft seals are thus used, a large friction loss occurs when the ball nut rotates, there are caused a loss of driving torque and an unagreeable feeling of the steering. Further, the steering gear case and the ball nut need high-precision working of seal seats and seal slide surfaces in order to accommodate and hold the shaft seals. This is a factor for increasing the costs for the product along with the rise in the number of components.